1. Field of the Invention
This invention relates to optical transceivers used in free space optical communications systems.
2. Description of the Related Art
There is a rapidly expanding need for data transmission. For example, while the use of fiber optics has increased the capacity and efficiency of data transmission, expanding data transmission needs require continual additions to the fiber optics infrastructure at significant expense and difficulty. One approach to augmenting the data transmission system is through the use of free space optical communications systems, which transmit light waves through the free space of the atmosphere.
U.S. Pat. No. 6,721,510, “Atmospheric Optical Data Transmission System” by J. Elon Graves and Malcolm J. Northcott discloses various free space optical communications systems that use adapative optics to compensate for atmospheric conditions. In many free space optical communications systems, a shared telescope is used to both transmit an outgoing optical beam and to receive an incoming optical beam. For example, the outgoing optical beam may be used to transmit data to a distant location while the incoming optical beam is used to receive data transmitted from the distant location. For many reasons, it can be advantageous for the outgoing and incoming optical beams to share largely the same free-space optical path within the telescope.
However, at some point, the two optical signals must be separated. An optical circulator is a nonreciprocal device that is commonly used for this purpose in fiber optic communications systems. An optical signal that enters port 1 of the optical circulator exits from port 2, but an optical signal that enters port 2 does not exit from port 1. It exits from a different port 3 instead. However, most, if not all, optical circulators that are used in fiber optic communications networks are symmetric. All of the ports typically are single mode fibers. Along the transmit optical path, the optical circulator couples light from the input port 1 single mode fiber to the port 2 single mode fiber to be transmitted as an outgoing optical signal to another fiber node. Along the receive optical path, the optical circulator couples an incoming optical signal from the port 2 single mode fiber to the output port 3 single mode fiber. The symmetric design is generally preferred for fiber optic communications networks because single mode fibers are generally preferred due to their performance.
However, this approach usually is not suitable for free space optical communications systems. In these systems, the light that is transmitted from a transceiver may have a well-defined Gaussian shape and therefore may arrive via a single mode fiber. However, due to effects such as aberrations along the free space optical communications link and overfilling of the receive aperture, the light received by a transceiver typically does not have such a tightly controlled mode and cannot be efficiently coupled back into a single mode fiber.
U.S. Pat. No. 7,194,159, “Asymmetric Optical Circulator” by J. Elon Graves discloses an asymmetric optical circulator that addresses the asymmetry introduced by free space optical communications systems. In the asymmetric optical circulator, input port 1 is a single mode fiber, port 2 is a free space port (i.e., not coupled to a fiber) coupled to the rest of the telescope, and output port 3 is a multimode fiber. In this way, along the transmit path, a single mode optical signal can be transmitted via single mode fiber port 1 and free space port 2. At the same time, a more distorted (i.e., multimode) optical beam can be received via free space port 2 and multimode fiber port 3. However, since port 2 of the optical circulator couples directly to the rest of the telescope, this approach requires precise alignment of the optical circulator with the rest of the optical train of the telescope.
Therefore, there is a need for improved approaches to coupling input and output signals to free space optical communications systems.